In roofing shingle applications, there is a need for increased flexibility to improve wind damage resistance and increased impact resistance to improve hail damage resistance. Previous improvements involving styrenic block copolymer/asphalt mixtures have made advances in this area. U.S. Pat. No. 4,405,680 taught polymer modified shingles where a blend of an unblown, compatible asphalt and an SBS styrenic block copolymer was made. The glass mat substrate of the shingle was impregnated with this blend. However, in order to achieve the required durability, the shingle was coated with a relatively high penetration value (PEN), blown asphalt.
In the blowing process, asphalt is oxidized by passing an oxygen-containing gas through the asphalt. This process serves to increase the asphaltene content, introduce ester functionality, and increase molecular weight. These are all factors which lead to incompatibility with styrenic block copolymers. Thus, while the blowing operation does increase the stiffness and softening point of the asphalt, it also makes the asphalt less suitable for blending with styrenic block copolymers. Blends of styrenic block copolymers and blown asphalt generally have poor storage stability with the block copolymer separating from the blend over time. Further, the conditions for blowing are harsh in that high temperatures and highly oxidative conditions are employed. This leads to severe degradation of SBS-type polymers.
However, methods have been developed, as taught in U.S. Pat. Nos. 5,939,474 and 6,060,542, in which blends of styrenic block copolymers and asphalt are blown. These methods require a blowing catalyst in order to conduct the blowing operation at conditions suitable for block copolymers. Even with mild conditions and blowing catalysts, though, hydrogenated SEBS-type block copolymers have been found to be unsuitable, as taught in U.S. Pat. No. 5,342,866.
A need yet exists for stable high polymer content blown polymer modified bituminous compositions and processes for preparing them which do not require blowing catalysts. Such an improvement would yield shingles with excellent in-use performance and excellent resistance to mechanical damage such as scuffing during roof installation and repair using simple and economical processes. The present invention provides a method and a material suitable for block copolymer modified shingle manufacture comprising a blown mixture of asphalt and an SEBS-type block copolymer which has a high PEN value and maintains its flexibility and impact resistance.